(1) Field of the Invention
The invention relates to the general field of chemical vapor deposition, more particularly to the problem of reducing particulate contamination.
(2) Description of the Prior Art
FIG. 1 is a schematic representation of an apparatus for performing low pressure chemical vapor deposition (LPCVD), typical of the type used in the manufacture of integrated circuits. Semiconductor wafers 1 are stacked in quartz boat 2 and placed inside heated reaction chamber 3 which is then sealed by securing end plate 4. Reactive gas may be admitted to the chamber through inlet valves 10. The other end of the reaction chamber is connected to a vacuum system comprising a pump 5, vacuum line 6 (which extends from the pump to the reaction chamber), trap 7, gate valve 8, and automatic pressure control valve 9.
In order to maintain the highest possible level of cleanliness, particularly in relation to dust particles of all sorts, the semiconductor wafers are prepared for the LCVD operation in clean room 11, separated from the pumping system by barrier 12. Despite these precautions, one of the disadvantages associated with LCVD has been reported to be "" susceptibility to particulate contamination"". See, for example, S. Wolf and R. N. Tauber in Silicon Processing for the VLSI Era, vol. 1, page 169. This susceptibility to contamination exists despite the presence of trap 7 whose purpose is to filter out any contaminant matter that might flow back from the pump into reaction chamber 3.
For example, during the deposition of silicon oxide from the decomposition of tetra ethyl ortho silicate (Si(OC2H5)4), only about 40% of the reactants are decomposed in the heating chamber, the remaining 60% being pumped out into the vacuum line. As a result, a goodly fraction of the reaction byproducts end up being deposited there and can be fed back into the main reaction chamber despite the presence of trap 7.
We are not aware of any prior art that addresses this specific problem, i.e. particulate contamination that is specifically associated with the LPCVD process.
It is therefore and object of the present invention to provide a process for performing LPCVD that results in essentially no contamination from particulate matter.
A further object of the present invention is provide the apparatus with which to achieve the first object.
Yet another object of the present invention is to provide both a method and an apparatus, for performing LPCVD with essentially no contamination from particulate matter, that add little or nothing to the costs associated with performing LPCVD under the current art.
These objects have been achieved through several modifications to the LPCVD equipment of the prior art. A bypass vent has been added in parallel with the main vacuum exhaust gate valve. Said bypass vent is left open during loading and unloading of the system with wafers that are to be processed, thereby ensuring a steady flow of air away from them at all times. Additionally, the section of the vacuum line immediately adjacent to the reaction chamber is heated.